Semiconductor device fabrication often involves deposition of tungsten-containing materials to be used for horizontal interconnects, vias between adjacent metal layers, and contacts between first metal layers and devices on the silicon substrate. In a conventional deposition process, tungsten is deposited in a chemical vapor deposition (CVD) process by exposing the substrate to a tungsten-containing precursor and a reducing agent.
However, as devices shrink, features become narrower and aspect ratios increase dramatically resulting in various challenges in depositing tungsten. As a result, alternative materials are sought to fill features. Cobalt is considered a promising candidate because of its low electron mean free path and good electro-migration performance. However, current processes for depositing cobalt have various challenges including non-conformal deposition in features, which can cause one or both of overhang on a feature entrance and insufficient coverage on the bottom of the feature. Non-conformal deposition can lead to voids in the feature.